Diesel engines produce various undesirable combustion byproducts including nitrogen oxides (NOx) and particulate matter (PM). As these byproducts have a negative effect on the environment, the Environmental Protection Agency (EPA) has imposed various regulations over the years designed to reduce their emission. These regulations apply to off-road diesel engines and stationary engines. Recently, the EPA graduated its emissions regulations for large stationary generator (genset) systems to the Tier 4 Interim (Tier 4i) requirement, which will be followed in 2015 by the even more stringent Tier 4 Final (Tier 4F) requirement. One technology for treating the exhaust stream from diesel engines in a system designed to meet these requirements is Selective Catalytic Reduction (SCR).
SCR is an after-treatment technology designed to permit NOx reduction reactions to take place in an oxidizing atmosphere, thereby chemically washing out the NOx from the exhaust before the exhaust is released into the environment. In general, an automotive grade urea-based solution (called diesel exhaust fluid (DEF) in North America) is injected into the exhaust upstream of a catalyst. The DEF decomposes to form ammonia (NH3) which, with the SCR catalyst, reacts with the NOx and converts it into nitrogen, water, and small amounts of carbon dioxide (CO2), all natural components of air.
As indicated above, SCR technology is an after-treatment process. If the SCR system is not functioning properly, unacceptable emission levels will result as the engine continues to produce NOx. Thus, while the technology is effective, it is only as effective as the approach implemented for maintaining optimum operation. One challenge to ensuring an SCR system continuously functions as intended is addressing the need to maintain the DEF supply at an acceptable level. Engine maintenance personnel need to be alerted when DEF supplies are low so they can take action to refill the DEF tank. Moreover, in some instances engine operators intentionally substitute DEF with a watered down version (or even pure water) to reduce costs. Unless a sufficiently high quality DEF is used, the NOx removal function of the SCR system is degraded or even eliminated, and the result is excessively high emissions. Additionally, engine operators may attempt to tamper with or skip required maintenance on certain components of the after-treatment system and thus override emission reduction and safety functions. Consequently, the EPA has issued guidelines requiring strategies for inducing engine operators to maintain proper function of the Tier 4 after-treatment system, such as maintaining the proper DEF supplies necessary to keep the SCR systems functional and to refrain from intentional tampering.